Prince William and Harry remember 'naughty' Diana's warmth and humour

Britain's Prince William and Harry have paid tribute to their mother Princess Diana, and revealed they regret rushing a phone call with her on the day she died, in a new documentary about her life.

William remembered the "warmth" and "humour" of his mother, and the brothers shared never-before-seen images of their family from Diana's personal photo album.

"Twenty years on Harry and I felt that it was an appropriate time to open up a bit more about our mother," he said.

The brothers revealed they talked to their mother on the day she died and that the "short" conversation now weighs "heavily" on their mind.

"It was her speaking from Paris, I can't really necessarily remember what I said but all I do remember is probably regretting for the rest of my life how short the phone call was," Harry said of the call.

Prince William was 15 years old and his brother Harry 12 when their mother and her boyfriend Dodi Al-Fayed were killed in August 1997 in a car crash in a tunnel in central Paris. The car crashed as it was being pursued at high speed by press photographers.

Their French chauffeur Henri Paul, who was later found to be over the legal blood alcohol limit, also died.

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At the time, the two princes were in Balmoral, the queen's residence in the Scottish Highlands, and William had earlier told Diana of the "very good time" they were having.

"Harry and I were in a desperate rush to say goodbye, you know 'see you later'...if I'd known now obviously what was going to happen I wouldn't have been so blase about it and everything else," William told ITV.

William said the ITV documentary will be the first and last time he and Prince Harry will speak candidly about their mother.

"We won't be doing this again - we won't speak as openly or publicly about her again, because we feel hopefully this film will provide the other side from close family friends you might not have heard before, from those who knew her best and from those who want to protect her memory, and want to remind people of the person that she was," he said.

"The warmth, the humour and what she was like as a mother, which probably hasn't come across before in many other pieces and from other people."

Prince William and Prince Harry sit on a picnic bench together. (Kensington Palace)

The program, 'Diana, Our Mother: Her Life and Legacy', also features the Princess' brother Earl Spencer and friends like Elton John.

The 20th anniversary of her death will be marked with a statue of Diana erected in a place she knew well, the public gardens of her former home Kensington Palace.

"Harry and I feel very strongly that we want to celebrate her life and this is a tribute from her sons to her, and we want her legacy to live on in our work and we feel this is an appropriate way of doing that," William said.

"To remind not only people who knew her, but also you have to remember this is 20 years ago now she died and there are people who don't even know about her.

"And I think it's been quite cathartic for us doing it. It's been at first quite daunting - opening up so much to camera...but going through this process has been quite a healing process as well."